ACTIVATE the TWO-FACTOR Authentication for a User

Switch to the user who should use the two-factor authentication and type in:

google-authenticator

You will be prompted to answer a few questions; Just answer all the questions with yes (y): Or depends on your preferred setup.

burnz@bastionsg:~$ google-authenticator
Do you want authentication tokens to be time-based (y/n) y
https://www.google.com/chart?chs=200x200&chld=M|0&cht=qr&chl=otpauth://totp/burnz@bastionsg%3Fsecret&8667677JSHKF
BAR CODE DISPLAY HERE
BAR CODE DISPLAY HERE
BAR CODE DISPLAY HERE
BAR CODE DISPLAY HERE
Do you want me to update your "/home/burnz/.google_authenticator" file (y/n) y
Do you want to disallow multiple uses of the same authentication
token? This restricts you to one login about every 30s, but it increases
your chances to notice or even prevent man-in-the-middle attacks (y/n) y
By default, tokens are good for 30 seconds and in order to compensate for
possible time-skew between the client and the server, we allow an extra
token before and after the current time. If you experience problems with poor
time synchronization, you can increase the window from its default
size of 1:30min to about 4min. Do you want to do so (y/n) y
If the computer that you are logging into isn't hardened against brute-force
login attempts, you can enable rate-limiting for the authentication module.
By default, this limits attackers to no more than 3 login attempts every 30s.
Do you want to enable rate-limiting (y/n) y

To use your mobile Google Authenticator Apps simply scan the bar code from the link it creates

Finally restart the SSH server.

burnz@bastionsg:~$ sudo /etc/init.d/ssh restart

Try to remote login to the server via SSH and verify that its working fine.

login as: burnz
Using keyboard-interactive authentication.
Password: # This is where the default password of the user
Using keyboard-interactive authentication.
Verification code: # This is where the code from your mobile google authenticator
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Debian GNU/Linux comes with ABSOLUTELY NO WARRANTY, to the extent
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You have mail.
Last login: Wed Apr 27 15:07:37 2016 from 10.2.1.102
burnz@bastionsg:~$ # Shows that you are successfully logged in